Many small scale liquid dispensing devices, sometimes called micro-fluidic devices, are known. These devices include micro-electromechanical system (MEMS) devices, electrical semiconductor devices, and others. These devices are small, typically in the range of 500 microns down to as small as 1 micron or even smaller. These devices are important in a wide range of application that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and marking technologies including a range of ink jet technologies, such as thermal ink jet and piezoelectric ink jet. Many of these devices have one or more displaceable devices, sometimes called actuators, which physically fluctuate to move fluid through the liquid dispensing device. These actuators typically include a material that responds to an electrical signal by expanding or contracting. For example, piezoelectric materials, such as lead-zirconium-titanate (PZT), may be sandwiched between two electrodes. In response to one of the electrodes receiving an electrical signal, an electrical field is established between the two electrodes and the PZT material physically moves. By positioning an actuator adjacent to a flexible membrane that follows the movement of the actuator, the flexible membrane is induced to move and expel liquid from a supply located next to the flexible membrane. In devices having multiple actuators, movement by one actuator may induce movement in a structure associated with another actuator. Consequently, the operation of the influenced actuator may be adversely impacted.
Modern printers incorporate printheads having a plurality of actuators that operate in a manner as described above to eject ink drops onto an imaging surface. The liquid ink may be stored in reservoirs installed into the printer or solid ink may be loaded as blocks or pellets into an ink delivery system. The delivery system transports the solid ink to a melting device where the solid ink is heated to a melting temperature and the melted ink is then collected. In both types of printers, the liquid ink is delivered to a printhead for ejection in a controlled pattern to generate an image.
The ejected ink is received on an imaging surface advancing past the print head. The imaging surface may be some form of media or an offset imaging member. In offset printing, the image is typically generated on a rotating offset member and subsequently transferred to media by synchronizing passage of media and rotation of the image on the member into a transfer nip formed between a transfix roller and the offset member. The printheads for liquid ink and solid ink printers typically include a plurality of micro-fluidic devices, called ink jet stacks, which are arranged in a matrix within the print head. Each ink jet stack has an array of nozzles from which ink is ejected by applying an electrical driving signal to one of the actuators in the array in the ink jet stack to generate a pressure pulse that expels ink from an ink supply in the ink jet stack.
A partially assembled ink jet stack is shown in a cross-sectional side view in FIG. 13. The ink jet stack 10 may be comprised of a number of plates that are mounted to one another. For example, the ink jet stack 10 may include a nozzle plate 14, an inlet plate 18, a body plate 22, and a diaphragm plate 26. These plates are assembled and bonded to one another using brazing or adhesives in a known manner to form ink jet stack 10. Additionally, other layers, such as filters, heating layers, or the like, may be included in the stack. Alternatively, an ink jet stack may be made from subassemblies, some of which are molded or formed by other processes, such as lithography or etching. These ink jet stacks may be formed with a structure similar to the one shown in FIG. 13. Regardless of the fabrication method, ink supplies 34 receive ink from an ink source through inlets 38. In response to the input of electrical energy provided through conductive adhesive 46 and an electrical contact pad 50, actuator 42 moves to induce movement in the diaphragm 26 mounted to the actuator. The diaphragm plate 26 is made of a resilient, flexible material, such as stainless steel, which enables the plate to move back and forth to expel ink in one direction of movement and to induce movement of ink into the ink supplies 34 in the other direction of movement. The ink expelled from an ink supply 34 exits through one of the openings 30 in the nozzle plate 14.
The electrical contact pad 50 is mounted to a support member 54, such as a flex cable or a multi-layer circuit board, which is partially supported by standoffs 58, which are also mounted to the support member 54. The actuator 42 may include piezoelectric material, such as lead-zirconium-titanate (PZT), which is sandwiched between two electrode structures, which may be made of nickel, for example. An electrical signal generated by a printhead controller is conducted by an electrical lead integrally formed with the electrical contact pad 50 to the conductive adhesive and the electrode contacting the adhesive. The charge on the electrode results in an electric field between the two electrodes on opposite sides of the PZT material. In response to the electric field, the PZT material deflects as shown in FIG. 14 and moves the diaphragm as well. In FIG. 14, the actuator material 42a and the portion of the diaphragm 26a immediately adjacent to the actuator material has moved to induce ink to be pulled into the ink supply 34a, while the actuator material 42b and the portion of the diaphragm 26b immediately adjacent that actuator material is moved to expel ink from the ink supply 34b. Thus, a printhead controller selectively generating an electrical signal is able to cause an ink jet stack to eject ink in an on-demand manner.
As may be discerned from FIG. 14, the deflection of the actuator material produces a force that primarily acts upon the diaphragm to expel ink from or pull ink into an ink supply 34. This force also operates on the conductive adhesive 46, the electrical contact 50, and the support member 54. This operation is a mechanical load and a parasitic force on the electrical connections that decreases the deflection of the actuator and, hence the force available for manipulation of the ink. In known printheads, this parasitic action is compensated by increasing the voltage for the driving signal, the size of the actuator, or other alterations of the stack structure. The size of the conductive adhesive, which may be silver epoxy or z-axis conductive tape, for example, the distance between the actuator material 42 and the contact pad 50, and the location of the conductive adhesive with respect to each actuator and pad, are all factors that can vary across the array of ink jets in a printhead. These variations, in turn, cause the mechanical loads on the actuator materials and the corresponding driving signal voltages that generate similarly sized ink drops from each ink jet to vary as well. These driving signal voltages that generate similarly sized ink jets are determined in a process known as normalization. These voltages are stored in a memory and later retrieved and used by the printhead controller to operate the ink jets in a printhead.
Determining the different driving voltages in a normalization process requires an application specific integrated circuit (ASIC), additional memory, and a portion of the printer set-up. Additionally, compensating for the differences between the ink jets in a printhead adds to the overhead for operating a printhead. As the number of nozzles in a printhead increases, these costs also increase. Thus, decreasing the differences between the structure of ink jet stacks for individual jets is worthwhile. Additionally, the transmission of shear stress from one ink jet to another through the support member also impacts the operation of the ink jets and may result in mechanical cross-talk. Such cross-talk may render an actuator's performance dependent upon whether neighboring actuators are being actuated. Moreover, cyclic stresses caused by the repeated deflections of the actuator material may, depending on the particular geometry of the structure in a particular ink jet, lead to damage to the actuator, the electrical contact pad, and/or the conductive adhesive. Consequently, more uniform ink jet body structure is desirable.